Generally, prior antenna apparatuses mounted in vehicles can receive AM broadcasts and FM broadcasts. In the prior antenna apparatuses, rod antennas having a length of about 1 m have been used to receive the AM broadcasts and the FM broadcasts. The lengths of the rod antennas become almost quarter-wave in the FM wavebands while becoming extremely short with respect to the wavelengths in the AM wavebands, thereby extremely reducing the sensitivity of the rod antennas in the AM wavebands. Therefore, conventionally, the sensitivity has been secured in the AM wavebands so as to enhance the impedance of the rod antennas using high-impedance cables, or so as to amplify signals with AM-waveband amplifiers. There have been also used in-car helical antenna apparatuses in which rod portions of the antennas are wound in helical shapes to shorten the antenna lengths to about 186 mm to about 400 mm. However, the amplifiers are provided immediately below the antennas in order to compensate performance degradation caused by the shortened rod portions.
FIG. 70 is a side view illustrating a configuration of a prior antenna apparatus 100 in which a rod portion is shortened. The prior antenna apparatus 100 of FIG. 70 includes an element 110 and an antenna basis 111 in which a lower end of the element 110 is rotatably mounted within a predetermined angle range. The antenna basis 111 includes an antenna cover and an antenna base. An amplifier and a matching circuit are incorporated in the antenna cover, and the antenna base is fitted in a lower surface of the antenna cover. A bolt portion is projected from the lower surface of the antenna base in order to mount the antenna apparatus 100 in a vehicle body. The antenna apparatus 100 receives AM broadcasts and FM broadcasts, the element 110 has a length of about 180 mm, and a total height h100 from the lower surface of the antenna basis 111 to a leading end of the element 110 is set at about 195 mm. The element 110 includes a helical element and an element cover with which the helical element is covered.
FIG. 71 is a side view illustrating the state in which, in the antenna apparatus 100, the element 110 is rotated in a direction perpendicular to the antenna basis 111 to set a height h101 from the lower surface of the antenna basis 111 to the leading end of the element 110 at about 70 mm.
Patent Document 1
Japanese Publication Unexamined Patent Application No. 2005-223957
Patent Document 2
Japanese Publication Unexamined Patent Application No. 2003-188619